Density-dependent effects on tree survival in an old-growth Douglas fir forest

1 We mapped the locations of live and dead trees in a large forest plot dom inated by pioneer Douglas fir (Pseudotsuga menziesii) with an understorey o f the invading late-successional species western hemlock (Tsuga heterophyll a) and western red cedar (Thuja plicata) on Vancouver Island, British Colum bia, Canada, to test for intra- and interspecific density-dependent effects on tree survival. 2 We analysed both the spatial patterning of trees in the plot and the rela tionships between neighbourhood density and tree survival. We also examined the effects of additional variables (principally elevation) as covariates in our neighbourhood analyses, 3 Both the spatial and initial neighbourhood analyses suggested strong intr a- and interspecific density-dependent effects on tree survival, Douglas fi r survival was significantly higher in less dense patches of conspecifics a nd non-random tree death led to regularly spaced survivors, as expected fro m intraspecific competition. The significantly lower survival of western he mlock in denser patches of Douglas fir and the resulting negative spatial a ssociation between surviving trees of these two species were consistent wit h interspecific competition. 4 However, having controlled for the influence of elevation on tree surviva l (probably mediated by variation in soil moisture) in neighbourhood analys es, although the survival of the pioneer Douglas fir trees was still subjec t to strong density-dependent effects., variation in its density in the ove rstorey no longer appeared to influence the survival of the invading late-s uccessional species, There was, however, evidence for asymmetric interspeci fic density dependence between the two late-successional species since west ern hemlock mortality tended to be higher in denser patches of western red cedar. 5 Our results emphasize the importance of considering confounding factors i n studies that seek evidence for density dependence.